Polarization Conversion Efficiency Research Articles

Ultrathin Bi‐Switchable Vanadium Dioxide‐Based Multifunctional Metamaterial for Terahertz Polarization Modulation

AbstractA broadband multi‐functional bi‐switchable ultrathin optical metamaterial design is investigated, which can achieve a high polarization conversion efficiency over a wide band of frequencies in the terahertz (THz) regime. The proposed design switches from a linear and circular polarization converter (LPC/CPC) to an efficient absorber by using the phase change property of a functional material, vanadium dioxide (VO2). The proposed design shows switching from an absorber to a reflector using the mechanical configuration of the functional material, and therefore the design can be used as a bi‐switchable device. The first type of switching is accomplished by changing the conductivity of the functional material, while the second type of switching is achieved with the help of the mechanical configuration of the functional material. Moreover, the structure is responsive to wide incident angles. The proposed switchable design has the potential to contribute to the development of tunable polarization rotating devices and on/off switching LPC devices, which have wide applications in detection, sensing, and communications.

Cross-connection of multiplexed cylindrical vector beams using off-axis spin-decoupled metasurfaces.

Cylindrical vector beam (CVB) multiplexing communication demands effective mode cross-connection techniques to establish communication networks. While methods like polarized grating and coordinate transformation have been developed for (de)multiplexing CVB modes, challenges persist in the cross-connection of these multiplexed mode channels, including multi-mode conversion and inhomogeneous polarization control. Herein, we present an independent off-axis spin-orbit interaction strategy utilizing spin-decoupled metasurfaces. Cross-connection is achieved by encoding conjugated Dammann optical vortex grating phases onto the two orthogonal circularly polarized components of CVBs. Experimental results demonstrate the successful interconversion of four CVB modes (CVB+1 and CVB-2, CVB+2 and CVB-4) using a Si-based metasurface with a polarization conversion efficiency exceeding 85%. This facilitates the cross-connection of 200 Gbit/s quadrature phase-shift keying signals with bit-error-rates below 10-6. Offering advantages such as ultra-compact device size, flexible control of CVB modes, and multi-mode parallel processing, this approach shows promise in advancing the networking capabilities of CVB mode multiplexing communication networks.

High-efficiency reflective terahertz cross-polarization converter with broadband and wide-angle response

In this paper, a broadband terahertz metasurface dedicated to cross-polarization conversion was designed, fabricated, and assessed. The metasurface, comprising two nested double-split rings, features an inherent insensitivity to the angle of incidence. Simulations reveal that the converter achieves a >99% polarization conversion efficiency across the 90–140 GHz range. Moreover, it maintains a >90% polarization conversion ratio (PCR), even at a 50° incidence angle. The sample, featuring 50×70 arrays, was fabricated, and the relevant experimental results align closely with the simulated outcomes. The metasurface characteristics can markedly enhance the performance of cross-polarization converters operating in the terahertz range.

Effect of surface anchoring energy on a liquid crystal optical waveguide-based polarization rotator

This study reports the effect of the surface anchoring energy of a liquid crystal (LC) cell on the performance of the liquid crystal optical waveguide polarization rotator (LCOW-PR) for the purpose of providing a theoretical reference for practical preparation of the LCOW-PR. First, the expression for the deflection angle of the director at the boundary of the LC cell is derived so that the distributions of the director and dielectric tensor of the LC can be accurately solved under any anchoring energy. On this basis, the correlation between the crucial indicators such as the polarization conversion length (PCL) together with the polarization conversion efficiency (PCE) of the LCOW-PR and the anchoring effect strength is constructed by combining with the existing numerical algorithms. The numerical results show that the maximum variation of the PCL is lower than 0.1 µm as the anchoring effect strength increases from 1×10−6J/m2 to 1×10−3J/m2, while the PCE decreases from 99.72% to 78.33%. This implies that the PCL of the LCOW-PR does not depend on the surface anchoring energy, but the anchoring effect strength of the orientational layer must be controlled to the order of 10−6J/m2 or even lower to achieve high-performance conversion between the polarization modes. Simultaneously, the effectiveness of the calculations in this work is verified with the help of the coupled mode theory as well as a comparison to previous reports.

A metasurface-based full-color circular auto-focusing Airy beam transmitter for stable high-speed underwater wireless optical communications

Due to its unique intensity distribution, self-acceleration, and beam self-healing properties, Airy beam holds great potential for optical wireless communications in challenging channels, such as underwater environments. As a vital part of 6G wireless network, the Internet of Underwater Things requires high-stability, low-latency, and high-capacity underwater wireless optical communication (UWOC). Currently, the primary challenge of UWOC lies in the prevalent time-varying and complex channel characteristics. Conventional blue Gaussian beam-based systems face difficulties in underwater randomly perturbed links. In this work, we report a full-color circular auto-focusing Airy beams metasurface transmitter for reliable, large-capacity and long-distance UWOC links. The metasurface is designed to exhibits high polarization conversion efficiency over a wide band (440-640 nm), enabling an increased data transmission rate of 91% and reliable 4 K video transmission in wavelength division multiplexing (WDM) based UWOC data link. The successful application of this metasurface in challenging UWOC links establishes a foundation for underwater interconnection scenarios in 6G communication.

Multifunctional beam modulation based on gratings combined with additive metasurfaces

A design scheme combining grating and element additive metasurface is proposed in this paper. Multi-dimensional joint control of the phase, amplitude, angle and polarization and multi-functional integrated metasurface applications were realized by using the inherent characteristics of the light wave properties (polarization and wavelength). Firstly, suitable structural parameters are obtained based on the reflection response of the unit structure, and a 2-bit encoded metasurface array is constructed by utilizing the combinatorial operation between the unit structure. Combined with the polarization selectivity of the lower grating, the dual-function integration of asymmetric transmission (the contrast ratio is 98%) and abnormal reflections (operating bandwidth is 300 nm) was realized. Next, two new half-wave plates were obtained by addition and rotating the elemental structure, and the polarization conversion efficiency of 95%–97% was achieved thanks to the Fabry–Perot cavity formed by the upper metasurface and lower grating. Finally, encoded metasurface based on the geometric phase principle to achieve 300 nm bandwidth cross-polarized wave deflection characteristics with polarization conversion efficiency close to 100%. Our work combines grating structures with element additive metasurfaces, which solves the problem that element structure needs to be redesigned with the change of function, and also paves the way for the application of multifunctional integrated metasurfaces in optical sensing, optical imaging and other fields.

Focal length switchable metalens based on vanadium dioxide.

A metalens is a flat lens that can control the phase of light so that dispersed light can be reconcentrated. This study devised a tunable metalens with a switchable focal length based on the phase transition properties of vanadium dioxide (V O 2). The unit structure comprises three layers from bottom to top: gold, polyimide, and two square resonant rings. The metalens can not only transform incident x-polarized waves into y-polarized waves but also achieve beam focusing simultaneously. The designed metalens achieves polarization conversion efficiency at an operating frequency of 0.8THz. In the insulating state of V O 2, the beam focal point is at L=1914µm; in the metallic state, the wave converges at L=982µm, closely aligning with the predetermined focal length. By controlling external temperature, focal point switching can be achieved, making it highly versatile in practical applications.

Switchable visible and near-infrared light bifocal lens based on hybrid graphene-metasurface structures

We propose a wideband visible (VIS) and near infrared (NIR) light reflective flat lens with switching capability base on hybrid graphene-metasurface structure. This tunable flat lens structure consists of a slotted gold reflector substrate, a graphene monolayer and TiO2 nanofins that provide excellent focusing performance and tunability in the VIS and NIR range. In order to achieve the switching feature, a bi-functional device capable of switching reflection/absorption modes is required. To this end, the gold substrate with two cross slots has been used to create absorption properties and a graphene layer has been used to change the intensity of absorption and reflection. The switching property for incident light with circular polarization arises from uniformly changing the chemical potential of the graphene layer. By setting the chemical potential of graphene, μc, equal to 1.1 eV, we obtained the reflection property. The reflection mode exhibits a broadband (715–800 nm) optical response with high polarization conversion efficiency (more than 70 %). On the other hand, by choosing μc = 0.75 eV, the absorption feature is created at the wavelength of 700–780 nm. The reflected light can be focused on an arbitrary position above the structure by rotating the TiO2 nanofins with the help of Pancharatnam-Berry (PB) phase theory. In addition to obtaining the single and dual focus tunable flat lenses, we have also presented a multifunction configuration that can switch between different modes, such as non-focus, single and dual focus modes. Such switchable flat lenses can have various applications in imaging and sensing systems.

Design of a transmissive dual-frequency polarization rotator for linearly polarized electromagnetic wave

This paper presents the design of a transmission-type dual-frequency polarization rotator for linearly polarized electromagnetic wave. The rotator facilitates polarization conversion for incident waves at any azimuth angle ϕ at two specific frequency points, resulting in a transmission wave with a counterclockwise rotation of 2ϕ. The proposed polarization rotator structure comprises three metal layers separated by two dielectric substrates. It includes two large rectangular metal patches positioned orthogonally along the x- and y-axes, respectively, as well as two small rectangular metal patches also positioned orthogonally along the x- and y-axes, respectively. The upper and lower metal layers act as the receiving and radiation ends, respectively. The transmitting and receiving ends are interconnected through coaxial metal holes. Remarkably, polarization conversion efficiencies exceeding 0.95 and 0.94 at 12 and 17 GHz, respectively, are achieved, and these high conversion efficiencies remain consistent as the azimuth angle of incidence changes. Furthermore, experimental results obtained from fabricated samples align well with the simulation ones, thus validating the effectiveness of the proposed polarization rotator. The proposed transmissive dual-frequency polarization rotator exhibits good practical application prospects in mobile satellite communications, GNSS, WLAN, etc.

Dual-band polarization converter with antireflection-assisted all-dielectric metasurface

In this study, we demonstrate an all-dielectric metasurface that can efficiently manipulate polarization for two broadband polarization control in the infrared band. By breaking the correlation between phase delay and frequency, independent polarization control over two wavelengths (2 μm and 4 μm) can be achieved. The phase delay can be freely controlled by controlling the number of waveguide modes in the grating. When operating in single mode, the phase delay increases in a linear fashion with the thickness of the grating. While at two modes, the phase delay is periodically perturbed with increasing thickness, which is caused by the mutual interference between the two modes. Therefore, the phase delay of two independent bands can be freely controlled. Here, a free combination of a dual-band polarized half-wave plate and quarter-wave plate are designed. Half-quarter wave plate and quarter-quarter wave plate, are fabricated and experimentally demonstrated to have efficient broadband polarization conversion efficiency with relative bandwidths exceeding 42.8 %. Since only is the device made of pure silicon, we utilize micro-spheres structure on the back side as an anti-reflection layer, and the highest transmittance can reach about 87 %.

Ultra-broadband high-efficiency cross-polarization conversion and amplitude-phase manipulable multi-functional wavefront manipulation based on terahertz metasurface

Drawing upon the physical phenomenon of polarization transformation, this paper proposes an ultra-broadband, high-efficiency linear polarization converter composed of a metallic grating, an L-shaped metallic patch, and a dielectric substrate. The polarization conversion properties have been scrutinized using the finite element numerical simulation software CST. The computational outcomes reveal that the polarization converter operates within the frequency range of 0.5 THz to 1.8 THz, exhibiting a relative bandwidth of 113%, a transmission coefficient exceeding 0.87, a polarization conversion efficiency approaching 100%, and a phase coverage spanning 360°. Furthermore, a Fabry–Perot interference model was established utilizing Matlab to corroborate the concurrence between the theoretical analysis and the numerical findings. The polarization converter metasurface amalgamates both phase and transmission amplitude variations to accomplish not only a two-dimensional focusing lens operating between 1.55 THz and 1.65 THz, but also a spatial imaging capability utilizing transmission amplitude variation within the 0.5 THz to 1.15 THz range. The outcomes demonstrate that the devised metasurface exhibits ultra-broadband and high transmission efficacy, thus providing novel insights for the versatility of terahertz wave polarization and phase manipulation.

GaN vortex metasurface for interference and broadband characteristics.

We experimentally demonstrate a highly efficient metasurface-based optical vortex beam (OVB) composed of high-aspect-ratio gallium nitride (GaN) meta-structures with an exceptional simulated absolute polarization conversion efficiency (APCE) of up to 98%. A flower-like interference pattern emerges at the converging distance of the device with the helicity switching in spiral and dislocation interference patterns beyond this point, as confirmed through meticulous Mach-Zehnder interferometer analysis. The device also performs broadband capabilities across visible wavelengths. Experimentally demonstrated, the annular shape adeptly expands its diameter with increasing incident wavelengths. This phenomenon is rooted in the fascinating anomalous refractive and reflective characteristics inherent to subwavelength-period metasurfaces.

Comprehensive energy conversion efficiency analysis of micro direct methanol fuel cell stack based on polarization theory

This paper develops a novel analyzing method in order to describe the energy conversion process (ECP) of the micro direct methanol fuel cell (μDMFC) stack quantitatively. The method employs the Debye-Hückel ionic atmosphere theory to study the generation and loss of the energy during polarization coupling. The polarization and energy conversion efficiency (ECE) analyzing results show that high output power does not also mean high ECE of the μDMFC stack. After exceeding the corresponding output current (OC) of peak output power, the ECE falls significantly. The simulated ECEs of the proposed method are validated experimentally and the experimental results correlate well. Moreover, the presented method achieves the highest ECE of 27.96 % for the μDMFC stack fabricated in this research. The corresponding anodic methanol supply concentration (AMSC) and the OC are 2.7 M and 240 mA respectively. Compared with recent researches, the proposed method achieves an even more significant ECE improvement. The novel method of this research makes it possible to predict the highest ECE and the corresponding operating parameters, thus improving the ECE of the μDMFC stack accordingly. The practical application of μDMFC stack in powering portable devices for marine use will significantly benefit from the presented analyzing method.

Integrated gallium phosphide-waveguide polarization rotator based on rotating a horizontal slot by 45 degree for 700 nm wavelength

AbstractPolarization manipulation elements operating at visible wavelengths represent a critical component of quantum communication sub-systems, equivalent to their telecom wavelength counterparts. The method proposed involves rotating the optic axis of the polarized input light by an angle of 45 degree, thereby converting the fundamental transverse electric (TE0) mode to the fundamental transverse magnetic (TM0) mode. This paper outlines an integrated gallium phosphide-waveguide polarization rotator, which relies on the rotation of a horizontal slot by 45 degree at a wavelength of 700 nm. This will ultimately lead to the conception of a mode hybridization phenomenon in the waveguide. The simulation results demonstrate a polarization conversion efficiency of 99.99% (99.98%) for TE0-to-TM0 (TM0-to-TE0) mode conversion, with an extinction ratio of 46.14 (39.62) dB and insertion loss below 1.6 dB at the specified wavelength. Additionally, the fabrication tolerance with regard to the width, height, and half-beat length of the proposed structure is investigated.

Active anisotropic polarization conversion meta-surface for 6G communication bandgap in the reflection mode

This paper investigates a single-layered active anisotropic meta-surface design capable of achieving high polarization conversion efficiency in the 6 G communication band gap. Additionally, the designed structure can convert the linear polarization of the incoming wave fronts to its cross-polarization and linear polarization to circular polarization in the reflection mode. The polarization conversion characteristic is achieved due to the strong anisotropic behavior of the meta-surface. Such a simple design can achieve a 90% to 100% cross-polarization conversion efficiency over a frequency region of [0.28–0.36] THz. The linear to circular polarization conversion is performed near 0.26 THz and 0.40 THz. Between these two frequencies, band linear to cross-polarization conversion is more prominent. The proposed design can be tuned from a half-wave plate (HWP) or linear polarization converter to a quarter-wave plate (QWP) or circular polarization converter using the chemical potential of graphene. The optical response of the meta-surface behaves differently from different incident angles. The proposed polarization converter can contribute to the development of active polarization device.

Ultrawideband Tunable Polarization Converter Based on Metamaterials

In order to overcome the shortcomings of the traditional polarization conversion device, a symmetric “E” type metamaterial resonance cell structure is proposed in the paper for the design of polarization conversion devices. Two kinds of polarization converter models are designed in this paper, and their polarization coefficient, phase and polarization conversion ratio (PCR) are simulated and analyzed. The polarization coefficients at different incident angles and different phase transitions of VO2 are simulated and analyzed. The study indicates that the polarization conversion efficiency of the polarization converter reaches 90% in the 0–2.0 THz frequency band, and the performance can be maintained within the incident angle of 45 degrees, covering the whole X band, realizing the broadband line polarization conversion with extremely high polarization conversion rate. Moreover, the polarization performance is tunable with the phase transition of VO2, and the research results provide new ideas for the realization of ultrawideband tunable polarization conversion devices and the application of metamaterials.

A terahertz wideband linear-to-circular polarization converter based on meandering-structured metasurface

In the terahertz (THz) band, this paper proposes a transmissive linear-to-circular (LTC) polarization converter based on a bilayer metallic meandering-structured metasurface. In this design, the LTC polarization conversion is achieved by utilizing the meandering structure, and the bandwidth is greatly expanded by inserting a wire grid into the y direction of the back metal layer. Furthermore, this metasurface converter is characterized by its simple structure and ultra-thin thickness. In detail, the axial ratio of the proposed converter is less than 3 dB in the frequency range of 0.4–1.21 THz signifying 100% relative bandwidth. Meanwhile, the emulation polarization conversion efficiency exceeds 60% in the frequency range of 0.68–1.21 THz, and the experimental results are in general agreement with the simulation ones. The proposed converter provides a new idea for the LTC polarization conversion in a wide THz band.

A metasurface featuring multiple polarization conversions

This paper proposes a novel bow-tie metasurface that integrates various types of polarization conversions by optimizing traditional unit structures. The metasurface achieves wide-angle linear polarization conversion over the X-band (8–12 GHz) and has a linear polarization conversion efficiency of over 94% under normal incidence. In the Ku-band (12.4–15.6 GHz), the ellipticity value (e-value) and axial ratio calculations show that the linearly polarized waves incident along the Y axis will be effectively converted to left-handed circularly polarized waves. Additionally, in the X-band (8.9–10.5 GHz), the circular polarization and its co-polarization conversion efficiency can be maintained at 99.5%, allowing for an effective circular polarization phase shift using the Pancharatnam–Berry principle in this frequency range.

Multifunctional metamaterial for realizing absorption and polarization conversion on the basis of vanadium dioxide

In this paper, the phase change material vanadium dioxide (VO2) is introduced to propose a tunable multifunctional metamaterial device, which treats the phase change property of VO2 to realize the switching of absorption, linear to linear (LTL) polarization conversion, and linear to circular (LTC) polarization conversion functions. When VO2 is in metal state, the structure can be applied as an absorber to achieve the wide-band and narrow-band absorption. In the broad frequency area of 2.007–2.803 THz, the absorption is above 90%. And the narrow-band absorption near the center frequency 1.0126 THz reaches 95.04%. When VO2 is in the insulator state, the studied device can be treated as a polarization conversion device to achieve LTL and LTC polarization conversion. In the low frequency range, the device achieves LTL with polarization conversion efficiency (PCR) above 90% when the frequency is between 0.439 and 0.907 THz. In the high frequency range, the structure can be treated as the LTC polarization converter to achieve right-hand circular polarization, left-hand circular polarization (LHCP), and LHCP in the ranges of 2.317–2.329 THz, 2.356–2.531 THz and 2.582–2.633 THz, respectively. In addition, the ellipticity is above 0.9. Finally, the effects of geometric parameters, angle of incidence and polarization angle on the absorption and PCR are also discussed. The proposed structure has great potential for advanced technologies such as imaging, sensing, communication, and stealth.

A Metasurface-Based LTC Polarization Converter with S-Shaped Split Ring Resonator Structure for Flexible Applications.

This paper presents a metasurface-based linear-to-circular polarization converter with a flexible structure for conformal and wearable applications. The converter consists of nested S- and C-shaped split ring resonators in the unit cell and can convert linearly polarized incident waves into left-handed circularly polarized ones at 12.4 GHz. Simulation results show that the proposed design has a high polarization conversion rate and efficiency at the operating frequency. Conformal tests are also conducted to evaluate the performance under curvature circumstances. A minor shift in the operating frequency is observed when the converter is applied on a sinusoidal wavy surface.